1. Technical Field
The disclosure relates generally to nonvolatile memory devices and reading methods thereof. In particular, the disclosure relates to a nonvolatile memory device and a reading method thereof that are capable of reading a selected cell while compensating for interference from adjacent cells.
2. Related Art
Nonvolatile memory devices generally have memory cell arrays in which data are stored, and page buffers for use in reading data.
A memory cell array is usually composed of a plurality of strings. Strings are electrically connected to page buffers through bit lines. Each string is formed by a plurality of memory cells which are coupled in series.
Memory cells adjacent to each other may be affected by electromagnetic interference, especially in a programming operation.
For instance, when programming a second cell of a second string, a threshold voltage distribution of a first cell of a first string adjacent to the second string could be changed due to electromagnetic interference.
FIG. 1 graphically shows variations of the threshold voltage distribution of a cell (e.g., an EVEN cell) upon programming an adjacent cell (e.g., an ODD cell).
As shown in FIG. 1, classifiable distributions of threshold voltages of memory cells are divisionally grouped in an erased state PV0, a first programmed state PV1, a second programmed state PV2 and a third programmed state PV3. If there is electromagnetic interference between adjacent memory cells, the threshold voltage distributions respective to the programmed or erased states change and may cause data to be incorrectly read in the subsequent reading operation.
In addition, the current tendency is to scale down memory devices and memory cells in size to achieve a higher integration density. As a result, circuits elements of such memory devise and memory cells become closer to each other. In a nonvolatile memory device, since plural strings including memory cells are arranged with a very narrow interval in between, a memory cell can be easily affected by capacitive coupling with adjacent memory cells. For example, if a second cell of a second string adjacent to a first cell of a first string is programmed after programming the first cell of the first string, threshold voltages of the first cell of the first string may be changed due to electromagnetic interference (i.e., via capacitive coupling) due to the programming operation of the second cell of the second string.
When programming (arrow L in FIG. 1) a least significant bit (LSB) into the second cell, there is no considerable effect by such interference because the second cell will be further programmed with a most significant bit (MSB) in the subsequent step and then the programming operation is completed.
However, when programming an MSB into the second cell (e.g., the ODD cell in FIG. 1), a degree of interference to the adjacent first cell (e.g., the EVEN cell in FIG. 1) is variable in accordance with a level of a program voltage of the second cell. For instance, a change in threshold voltage is larger when programming the second cell into the first or third programmed state PV1 or PV3 (arrow M1 or M3 in FIG. 1) than when programming the second cell into the second programmed state PV2 (arrow M2 in FIG. 1). Accordingly, the first cell of the first string adjacent to the second cell of the second string is more affected when programming the second cell of the second string into the first or third programmed state PV1 or PV3 (M1 or M3) than when programming the second cell of the second string into the second programmed state PV2 (M2).
This may resultantly cause the programming reliability to be degraded in the nonvolatile memory device when the first cell, which has been inadvertently changed in threshold voltage distribution due to such interference, is sensed in a reading operation.